Amplified microwave power limiter



AMPLIFIED MICROWAVE POWER LIIIITER Filed Dec. 2, 1960 2 Sheets-Sheet 1 FER R lTE :2 I B r-- ---1 2 st 52. IT

FIG. 2

FIG. 3

F v I- 5 E l- :P 5 O 8 p 9 POWER INPUT r POWER mPu'r .1,

INVENTOR,

WILLIAM H. YR/6H7; JR.

dun Q5 A T TORNE X May 25, 1965 W. H. WRIGHT, JR

AMPLIFIED IICROW AVE POWER LIIII'I'ER Filed Dec. 2. 1960 2 Sheets-Sheet 2 IINVENTOR, WILLIAM H. wR/aHT, JR.

by W W? ATTORNEY United States Patent M 3,185,945 AMPLIFIED MICROWAVE POWER LIMITER William H. Wright, In, Wall Township, Monmouth County, N.I., assignor .to the United States of America as represented by the Secretary of the Filed Dec. 2, 1960, Ser. No. 73,461 6 Claims. (Cl. 333-81) (Granted under Title 35, US. Code (U52), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This invention relates to electromagnetic wave transmission systems and more particularly to a microwave power limiter for use in such systems.

It is well known that crystal detectors employed in radar receiver systems and other type microwave receivers are limited in the amount of power they can withstand without deteriorating. Of necessity, the power applied to such crystals must be relatively low and, for high incident power levels, a great degree of attenuation is desirable. Heretofore, power-sensitive non-linear elements such as ferrite subsidiary resonance absorbers, De- Grasse type ferrite limiters, or diode type limiters were utilized in transmission line sections to provide nonlinear attenuation characteristics. Such power limiters, however, do not provide enough loss, or a low enough threshold point for high power application.

It is therefore an object of the present invention to automatically control the level of power flow in a microwave transmission system.

It is another object of the present invention to provide an improved microwave power limiter wherein a non-linear attenuation is introduced in a travelling-wave ring-resonator.

It is another object of the invention to provide a microwave power limiter having an improved threshold power level.

In accordance with the present invention there is provided a microwave power limiter which includes first and second highly directive directional couplers each having four waveguide terminals and a source for supplying electromagnetic energy coupled to one terminal of the first coupler. Also included are output waveguide means coupled to one terminal of the second coupler, a first waveguide means coupled to two other corresponding waveguide terminals of the first and second coupler and a second waveguide means coupled to two other corresponding waveguide terminals oil the first and second coupler whereby a closed loop energy path is provided by the first and second directional couplers and the first and second waveguide means. The closed loop so formed is arranged to have an electrical length which is an integral number of guide wavelengths of the operating frequency. In addition, there are included discrete energy absorbing means coupled to each of the remaining terminals of the directionalcouplers and a non-linear power sensitive element mounted within the closed loop and responsive to the electromagnetic energy therein such that the output power derived therefrom increases linearly with power input until a threshold power is reached at which time the output power levels off and remains constant.

For a better understanding of the invention together with further objects thereof, reference is had to the following description taken'in connection with the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of the power limiter contemplated by the present invention;

FIGS. 2 and 3 are graphical curves used in explaining the operation of the power limiter; and

3,185,945 Patented May 25, 1965 FIG. 4 is a plan view of one embodiment of the present invention.

Referring now to FIG. 1 of the drawing, there is shown at 10 a re-entrant or closed-loop waveguide section commonly known in the art as a travelling-wave n'ng-resona tor which includes as part thereof two discrete'di-rec-Q tional couplers 12 and 14. Waveguide section 10 is coupled by means of directional couplers 12 and 14 to respective input and output rectilinear or rectangular; waveguides 16 and 18. The directional couplers 12 and 14 each have high directivity characteristics and each include two sections of hollow waveguide. As shown, directional coupler -12 includes hollow waveguide sections 20 and 22 having a directional coupling aperture 24 in the common wall between the sections of wave{ guides 20 and 22. The waveguide section 20 is connected at one end through rectilinear waveguide -16 to a microwave input source indicated schematically at 26, the other end of waveguide section 20 being terminated by a non-reflective load 28. Directional coupler -14 includes hollow waveguide sections 30 and 32 having a directional coupling aperture 34 in the common wall between sections of waveguides 30 and 32. The wave-1 guide section 30 is connected at one end through reeti Iline-ar waveguide 18 to an output circuit (not shown);

the other end of waveguide section 30 being terminated by a non-reflective load 36. The respective directional coupler waveguide sections 22 and 32 form part of the closed loop waveguide section 10. Each of the couplers may be said to comprise four ports labeled P, R, S, and T, respectively which correspond to the open-ended termination of the waveguide sections in each of the directional couplers. Thus the ports P and R of each directional coupler form part of the travellingwave ringresonator loop whose electrical length is an integral number of guide wavelengths of the operating trequency. One such directional coupler well known in the art is the Hewitt Packard multihole directional coupler X-752. It is to be understood of course that other suitable four. port couplers may be used such as the strip-line tour; port couplers also well known in the art. 1 Intermediate direction-a1 couplers 12 and 14 withini ring resonator 10 there is provided a non-linear power, sensitive element 88 having characteristics such that the? output power derived therefrom will increase linearly! with power input until a threshold power is reached,i at which time the output power levels olf and rcmainsi constant. One such power sensitive device may com-; prise a magnetically biased ferrite longitudinally positioned within a waveguide sect-ion which forms part of? ring resonator 10. This type of power sensitive device} is shown and described in Scovil Patent No. 2,920,292.; It is to be understood of course that other suitable power-i sensitive non-linear devices could be used. As shown,; the power sensitive non-linear device is positioned in-i termediate the directional couplers 12 and 14 within ring resonator 10 distal from the input and output ends of; respective rectilinear waveguides 16 and 18. Referring now to FIG. 4, where like numerals refer; to like components, there is shown a preferred embodiment of the invention. Directional couplers of high directivity are provided as indicated at 12 and 14. Directional coupler 12 includes a primary waveguide section; 20 and a secondary waveguide section 22 having their narrow or broad walls in parallel relationship. The input rectilinear waveguide 16 is connected to the input end ofj primary waveguide section 20 by means of a flanged con-=1 nection 42 and the load 28 is connected to the output end of primary waveguide section 20 by means of a fianged joint 44. The secondary waveguide section 22 is terminated at one end with a coupling flange 46 while the opposite end of secondary waveguide section 22 is similarly U terminated with a flange 48. Directional coupler 14 is identical in construction to directional coupler 12 and both include means (not shown) for varying the degree of coupling. Primary waveguide section 32 and secondary waveguide section 30 of directional coupler 14 have either their narrow or broad walls in parallel relationship. Primary waveguide section 32 is terminated at one end with a coupling flange 50 while the opposite end thereof is terminated with a flange 51. Secondary waveguide section 30 is connected at one end to output rectilinear waveguide 18 by means of a flanged connection 52 and the other end of secondary waveguide section 30 is connected to load 36 by means of flange connection 54. The closed loop path between secondary waveguide section 22 and primary waveguide section 32 includes the non-linear power sensitive element 38. In the embodiment shown, the non-linear power sensitive element includes waveguide section 56 having longitudinally mounted therein a ferrite 58 which is in the path of wave energy. Waveguide section 56 is connected by respective conventional flanged joints to the flanged termination of 48 and 50 of secondary waveguide section 22 and primary waveguide section 32, respectively. Although not shown, it is to be understood that ferrite 58 is magnetically biased or polarized by a magnetic field in the usual manner. The resonant closed loop is completed by a shim section of waveguide 60 terminated with conventional flanged joints which connect to flanged ,termination 46 and 51 of secondary waveguide section 22 land primary waveguide section 32, respectively. f In discussing the operation of the power limiter refer- Eence may be made .to FIG. 1 and the curves shown in FIGS. 2 and 3. In any directional coupling device having .Qhigh directivity, energy incident in one arm is divided between two of the other arms but does not couple to the fourth arm and when passing through or crossing the coupler aperture, the incident energy is shifted in phase by 90". It is to be assumed of course that the arms or waveguide sections of the directional couplers are all terminated in their characteristic impedance. 7

For convenience the phasor quantity voltages at the ports P, R, S, and T, of directional coupler 12 are designamd as EP12 L pm R12L R12 s12L s12 and 'rizL -112 respectively. Similarly the phasor voltages at the ports .P, R, S, and T of directional coupler 14 are designated as P14L Pt4 R14L R14 s14L s14 and 'rmL 'rlo spectively. It is understood of course that L0 denotes e which is standard phasor notation. Also, the coupling coefiicient of directional coupler 14 is designated as C and the coupling coefficient of directional coupler 12 is designated as C The transmission constant of one-half resonant ring may be represented as where a is the attenuation of one-half the ring circuit and the transmission constant of limiter element 38 may be represented as where B is the attenuation of limiter element 38. As is well known in the art, the respective maximum value of a, b, C and C is of course unity. With the above notations in mind, the relationship of the voltages at the ports of the directional couplers will be as follows:

where =90 which is the phase shift of the signal wave energy crossing the aperture in the respective directional coupler.

The relationship in Equations 1-4 are well known ex pressions for showing the amplitude relation between the incident voltage in one arm of a directional coupler and the respective output voltages from the two arms thereof. It can also be shown that Substituting the value of E in Equation 9 for E in Equation 8, we have ma L R12= 12 s1z L 512+ 12 From Equation 12 we have after collecting terms and from Equation 13 we have From Equations 3, 7 and 14, the following relationship may be derived:

From Equations 5, 6, 7, 11 and 14, the following relationship may be derived:

From Equation 16 it is seen that B 4 0 is composed of two components which differ in phase by 2o or 180 and hence are exactly out-of-phase. The phase can be denoted by changing the sign of one term as follows and thus eliminate the phase shifts entirely,

By imposing the condition of equal magnitude on these two out-of-phase components, complete destructive cancellation can be achieved. Setting E 1 40 equal to zero as follows Thus it is seen that if, by a proper choice of couplers 12 and 14 for given values of a and b, B will be identically zero, and that all incident energy, E will emerge as E less, of course, transmission losses within the ring. Thus, if ET12E0, Equation 19 may be substituted in Equation 14 so that which reduces to Emai E S 12 C12 Similarly, the relationship in Equation 15 can be shown to be 22 I E S12 C12 I In terms of power, Equation 21 can be written 1; 12 R-l2 i Thus the effective threshold level is decreased by the factor In other words, the power level presented to the limiting element 38 would be higher than that in the main line, thereby decreasing the effective threshold by the same amount. This has the same efiect as pushing the limit P in FIGURE 2 to the left as indicated by the broken line P. Also for high power level conduction, it can be seen that the maximum attenuation is increased by the decoupling factors so that the attenuation is now equal to the value bC C Moreover, at such high power levels, most power will be absorbed by resistor load 28 through port T thereby increasing power handling capacity by While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from :the invention, and it is, therefore, aimed in the appended claims to cover 'all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A microwave power-limiter comprising, rectilinear input and output waveguides each terminated at corresponding ends by energy absorbing means, a source supplying electromagnetic wave energy coupled to the other end of said input waveguide, a waveguide ring resonator, discrete means included in said ring resonator in respective coupling relationship with said input and output waveguides whereby said electromagnetic energy is coupled into said ring resonator from said input waveguide and electromagnetic energy is coupled out of said ring resonator to said output waveguide, the coupling means associated with the input waveguide having a prescribed coupling coefficient C, and a non-linear power sensitive element mounted within said ring resonator and responsive" to the electromagnetic wave energy therein such that the output power derived therefrom will increase linearly with power input until a threshold power is .reached at which time the output power levels off and remains constant, the power level in said waveguide ring resonator be- 6 ing above the power level of said energy source by the factor 2 A microwave power-limiter comprising, an input and output waveguide each being terminated at correeach having four waveguide terminals, a first and second waveguide means respectively interconnecting two corresponding waveguide terminals of each of said directional couplers whereby a waveguide ring resonator energy path is provided by each of said couplers and said Waveguide means, said input waveguide being electrically coupled to one of said directional couplers having a coupling coefiicient C, said output waveguide being electrically coupled to the other of said directional couplers, and a non-' linear power sensitive element mounted within said ring resonator in the path of the electromagnetic energy therein, the electromagnetic energy therein being at a power level above that of the power level of said electromagnetic energy source by the factor 3. A microwave power limiter comprising, a first and? second directional coupler each comprising a primary and% secondary waveguide, said second directional coupler hav-j ing a coupling coefiicient C, waveguide means electrically interconnecting said first coupler primary waveguide and! said second coupler secondary waveguide whereby a ringi resonator energy path is provided by said waveguidei means, said first coupler primary waveguide and said} second coupler secondary waveguide, a source of electro-j magnetic energy coupled to said second coupler primary waveguide whereby waves coupled into the ring resonator; propagate around said ring resonator, means coupled to said vfirst coupler secondary waveguide for extracting elec-i tromagnetic energy from said ring resonator, and a non-i linear power sensitive element mounted within said resonator in the path of electromagnetic energy therein, 5 the electromagnetic energy therein being at a power level; above that of the power level of said electromagnetic energy source by the factor 4. A microwave power limiter comprising, rectilinear input and output waveguides each terminated at corresponding ends by energy absorbing means, first and sec-t 0nd directional couplers each comprised of a primary and; secondary waveguide having a coupling aperture therebetween, said first directional coupler having a coupling co-'i efiicient C, first and second rectilinear waveguides in elec-' trical coupling relationship with said first coupler primary waveguide and said second coupler secondary waveguide} respectively, each of said rectilinear waveguides being coupled at one end to discrete energy absorbing means,i waveguide means electrically interconnecting said first coupler secondary waveguide and said second coupler primary waveguide to provide a waveguide ring resonator, a

source of electromagnetic energy coupled to the other end of said first rectilinear waveguide, and a non-linear power sensitive element mounted within said ring'resonator in the path of the electromagnetic energy therein, the electromagnetic energy therein being at a power level above terminals, said first directional coupler having a coupling coefiicient C, a source for supplying electromagnetic waves coupled to one terminal of said first coupler, output waveguide means coupled to one terminal of said second coupler, a first waveguide means coupled to two other corresponding waveguide terminals of said first and second couplers and a second waveguide means coupled to two other corresponding waveguide terminals of said first and second couplers whereby a waveguide ring resonator energy path is provided by said first and second couplers and said first and second waveguide means, discrete energy absorbing means being coupled to each of the remaining ;terminals of said directional couplers, and a non-linear power sensitive element mounted within said ring resonag tor and responsive to the electromagnetic energy therein ]-such that the output power. derived therefrom increases 5 linearly with power input until a threshold power is E reached at which time the output power levels off and remains constant, the power level in said waveguide ring iresonator being above the power level of said electrof magnetic wave energy source by the factor 6. The microwave power limiter in accordance with claim 1 wherein said power sensitive element is positioned within said waveguide ring resonator distal from the input and output ends of said respective rectilinear waveguides.

References Cited by the Examiner UNITED STATES PATENTS 2,652,540 9/53 Dietrich 333-14 2,652,541 9/53 Cutler .333-14 2,762,871 9/56 Dicke 333-10 2,875,415 2/59 Sferraz za 333-10 2,920,292 1/60 Scovil 333-24 2,922,964 1/60 Turner 333-24 2,930,004 3/60 Coale 333-10 2,936,430 5/60 Marie 333-10 2,951,217 s/eo Clapp $33-40 3,042,883 7/62 Pannenborg 333-14 3,049,679 8/62 Proctor 333-10 3,074,033 1/63 Smith 333-10 FOREIGN PATENTS 752,726 7/56 Great Britain.

ELI LIEBERMAN, Primary Examiner.

ELI I. SAX, HERMANN KARL SAALBACH,

Examiners. 

1. A MICROWAVE POWER-LIMITER COMPRISING, RECTILINEAR INPUT AND OUTPUT WAVEGUIDES EACH TERMINATED AT CORRESPONDING ENDS BY ENERGY ABSORBING MEANS, A SOUCE SUPPLYING ELECTROMAGNETIC WAVE ENERGY COUPLED TO THE OTHER END OF SAID INPUT WAVEGUIDE, A WAVEGUIDE RING RESONATOR, DISCRETE MEANS INCLUDED IN SAID RING RESONATOR IN RESPECTIVE COUPLING RELATIONSHIP WITH SAID INPUT AND OUTPUT WAVEGUIDES WHEREBY SAID ELECTROMAGNETIC ENERGY IS COUPLED INTO SAID RING RESONATOR FROM SAID INPUT WAVEGUIDE AND ELECTROMAGNETIC ENERGY IS COUPLED OUT OF SAID RING RESONATOR TO SAID OUTPUT WAVEGUIDE, THE COUPLING MEANS ASSOCIATED WITH THE INPUT WAVEGUIDE HAVING A PRESCRIBED COUPLING COEFFICIENT C, AND A NON-LINEAR POWER SENSITIVE ELEMENT MOUNTED WITHIN SAID RING RESONATOR AND RESPONSIVE TO THE ELECTROMAGNETIC WAVE ENERGY THEREIN SUCH THAT THE OUTPUT POWER DERIVED THEREFROM WILL INCREASE LINEARLY WITH THE POWER INPUT UNTIL A THRESHOLD POWER IS REACHED AT WHICH TIME THE OUTPUT POWER LEVELS OF AND REMAINS CONSTANT, THE POWER LEVEL IN SAID WAVEGUIDE RING RESONATOR BEING ABOVE THE POWER LEVEL OF SAID ENERGY SOURCE BY THE FACTOR. 